One of the protocols used in communications between network entities to provide a reliable, in-order delivery of information is the Open System Interconnection Reference Model (OSI Reference Model or OSI Model). Basically, the OSI model comprises a plurality of layers with each of the layers having functions that provide services to the layer above and receive service from the layer below.
More specifically, the OSI Model includes seven layers known as the: Application Layer or Layer 7; Presentation Layer or Layer 6; Session Layer or Layer 5; Transport Layer or Layer 4; Network Layer or Layer 3; Data Link Layer or Layer 2; and Physical Layer or Layer 1. The typical basic functions of each of these layers are described below. The Application Layer interfaces directly to and performs services for applications operating on the network entity and also issues requests to the presentation layer. The Presentation Layer is used to establish a context between application layer entities. The Session Layer controls the dialogues/connections or sessions between network entities including establishing check pointing, adjournment, termination, and restart procedures. The Transport Layer controls the reliability of a given link through error control. The Network Layer performs reassembly of packets. The Data Link Layer provides the functional and procedural way to transfer data between network entities. The Physical Layer establishes and terminates the actual connection to a communications medium.
A simplified example of a typical TCP communication will now be described below. Since the operation of and interaction between these layers are well known to those of ordinary skill in the art, the description herein will focus on the Application Layer or Layer 7; Transport Layer or Layer 4; Network Layer or Layer 3; and Physical Layer or Layer 1. A source network entity generates or otherwise obtains one or more packets with data to be transmitted to at least one other destination network entity. Each of these packets includes a header which includes information, such as the source, destination, sequence, acknowledgement, and checksum, and a body which includes information, such as the data or payload. The Network Layer performs the necessary routing for these packets to be transmitted from the source network entity through one or more networks to the destination network entity. Each of the transmitted packets can take a different route to the destination network entity and thus the packets may arrive at the destination in the wrong order.
The destination network entity receives the transmitted packets at the Physical Layer which are passed up to the Transport Layer as the necessary functions are completed. The Transport Layer checks the sequence number in each of the transmitted packets to make sure none of the transmitted packets were lost during the transmission. Based on the check at the Transport Layer, the destination network entity will send an acknowledgment for the transmitted packets which have been successfully received to the source network entity. If an acknowledgment is not received by source network entity within a reasonable round trip time, then the transmitted packet(s) without an acknowledgement(s) is/are presumed lost and are retransmitted.
Additionally, at the Transport Layer the destination network entity checks to see if any of the received packets are corrupted by using a checksum. If one or more of the packets are corrupted, then the corrupted packets are discarded and the source network entity is notified and retransmits the packet(s) which previously were corrupted. As the received packets are determined to be in the proper sequence and uncorrupted, they are passed up as the necessary functions in each of the layers are performed to the Application Layer. Accordingly, TCP is specifically designed to provide reliable and in order delivery of data between network entities. However, TCP does not provide for any reordering or cancellation of any packets once transmitted.